Algae

Algae are simple plants with chlorophyll that make their own food. Their body is a thallus (not divided into root, stem and leaf). Most live in water. They can be colonial (Volvox), filamentous like threads (Ulothrix, Spirogyra), or huge, like the kelps of the sea.

They reproduce in three ways. Vegetatively, by breaking into pieces (fragmentation). Asexually, by spores, mostly zoospores that swim with flagella. Sexually, in three patterns:

• Isogamy: the two gametes look alike (Ulothrix, Spirogyra).
• Anisogamy: the gametes differ in size (Eudorina).
• Oogamy: a large, non-moving female gamete (egg) fuses with a small, moving male gamete (Volvox, Fucus).

Algae matter a lot. They carry out at least half of all CO₂ fixation on Earth and are the base of water food chains. Some are eaten (Porphyra, Laminaria, Sargassum). Algin comes from brown algae and carrageen from red algae (a trick: carrageen has an "r", for red). Agar, used in jelly and for growing microbes in labs, comes from Gelidium and Gracilaria. Chlorella is even used as a protein supplement, including by space travellers.

Algae are divided by their pigments and the food they store:

• Green algae (Chlorophyceae): chlorophyll a and b; food stored as starch in pyrenoids; walls of cellulose and pectose. Chlamydomonas, Volvox, Spirogyra, Chara.
• Brown algae (Phaeophyceae): chlorophyll a and c and the brown pigment fucoxanthin; food stored as laminarin or mannitol. The body has a holdfast (grips the rock), a stipe (stalk) and a frond (leaf-like part); zoospores are pear-shaped with two flagella. Ectocarpus, Laminaria, Sargassum, Fucus.
• Red algae (Rhodophyceae): the red pigment r-phycoerythrin, which lets them use the light that reaches great depths of the ocean; food stored as floridean starch; spores and gametes do not swim; sexual reproduction is oogamous. Polysiphonia, Porphyra, Gracilaria, Gelidium.